Importance of Mitosis

The detailed stages of the cell cycle explain how this happens. Here, the important question is why organisms need the outcome. There are three central roles: growth, repair and asexual reproduction.

Growth by increasing cell number

A multicellular organism begins as a single fertilised egg cell, or zygote. That cell divides, and its descendants divide repeatedly. One cell becomes two, two become four, and continued mitotic division produces the enormous number of cells needed to form an embryo and then a larger organism.

Growth is therefore not simply a matter of every existing cell getting bigger. Cells do grow during the cell cycle, but mitosis allows the number of cells to increase. Many of the new cells can later differentiate, developing specialised structures for particular functions. Mitosis supplies the cells; differentiation gives them different roles.

Growth continues after early development. For example, mitosis produces additional cells as a child grows. In plants, actively dividing cells in meristems at the tips of roots and shoots provide new cells for growth. Later elongation and differentiation help those plant cells form new tissues, but mitosis is what increases their number.

Repair and replacement

Cells do not last for ever. Some are worn away, some die naturally and others are damaged by injury. Mitosis produces replacement cells with the same genetic information as the surrounding tissue.

For example, new cells help replace cells lost from the skin and gut lining. Following a cut, cells near the wound can divide, increasing the number of cells available to rebuild the damaged tissue. Cell division also contributes to the healing of broken bone and the continuing production of new blood cells.

Genetic similarity is important here: a replacement cell needs the same basic genetic instructions as the tissue it is restoring. After division, some new cells may differentiate so that they can perform the specialised function required by that tissue.

Repair has limits. Mitosis can replace cells only where suitable living cells remain able to divide and rebuild the tissue. It does not mean that every type of damage can be completely reversed.

Asexual reproduction

In asexual reproduction, a new organism is produced from one parent. There is no fusion of male and female gametes, so genetic information from two parents is not mixed.

The parent's DNA is copied before mitosis. Mitosis then separates the copied genetic material into new nuclei, allowing genetically identical cells to be produced. The offspring are therefore genetically identical to the parent and to one another, so they are described as clones. Plants can reproduce asexually through structures such as runners, and asexual reproduction also occurs in some fungi. Repeated mitotic divisions allow the new individual to grow.

This is different from sexual reproduction, in which gametes join during fertilisation and genetic information from two parents is combined. Sexual reproduction usually produces genetically varied offspring; asexual reproduction by mitosis usually preserves the parent's genetic information.

One process, three outcomes

The same basic ability to make genetically identical cells supports three different outcomes:

• growth: more cells are produced, increasing the size of a multicellular organism;
• repair and replacement: new cells replace damaged, dead or worn-out cells;
• asexual reproduction: mitosis contributes to producing a new organism from one parent, forming genetically identical offspring.

The connection is cell number. Mitosis increases the supply of cells without changing the chromosome number, whether those cells remain within the same organism for growth or repair, or contribute to a new organism during asexual reproduction.